1. Field of the Invention
The present invention relates to a method for electrically testing conductors.
It particularly, but not exclusively, applies to the electric test of substrates, such as semi-conductor chips or integrated circuits, and printed circuits.
2. Description of the Related Art
Current test machines measure the continuity of tracks and their insulation. For that purpose, they use tools with which it is possible to take or extract electrons from conductors. Among these tools, there is what is called “nail boards” that are specific to each type of circuit. It transpires that with the reduction in the dimensions of electronic components and the considerable increase in the density of printed circuit tracks, these tools are becoming more and more complex to produce, and therefore very expensive and less reliable. In addition, their manufacturing lead-time (from one to several days) is increasing, which is contrary to the greater manufacturing flexibility required by new generations of printed circuits (the shortening of production cycles imposes a heavy restriction on the manufacturing of these nail boards).
To take or extract electrons from conductors, there are also devices that use a high-energy source of coherent light, which, by photoelectric effect, leads to the rise in the voltage of these conductors. These tools are for example described in U.S. Pat. Nos. 4,837,506 and 4,967,152.
The photoelectric effect consists in the emission of electrons by a metal under the action of light radiation. More generally speaking, the photoelectric effect covers several phenomena of interaction between light and matter, in which photons transfer their energy to electrons. Thus, there is the external photoelectric effect, also called photoemission, and the internal photoelectric effect that comprises photoconductivity, the photovoltaic effect and photo-ionization.
It results in the absorption of certain photons by the metal: if the energy of a photon is higher than the energy linking an electron to one atom of the metal, that electron may then leave its atomic orbital, acquiring kinetic energy and creating an electric current.
In the photoelectric effect, the electrons are therefore ejected from a metal (or conductor) by the collision with incident photons. In fact, the electrons are kept inside the metal by a barrier of potential, called “Work Function” (WF). To enable an electron to be ejected, the energy of the photons must be higher than WF. For example, for copper, WF is approximately 4.3 eV (electron-Volts), and, as a result, the wavelength of the source of coherent light must be shorter than 1.24/4.3 =290 nm. Shorter wavelengths enable electrons to be released with non-zero kinetic energy.
The photoelectric effect does not depend on the intensity of the source of light. It can therefore be observed even for very low power used, which means that operation that does not damage the conductor can be considered. It does not occur at insulant level, even though ionization can occur under the action of highly energetic photons.
The ejected electrons can be collected by a positively biased electrode, and the current thus created can be measured. That requires a partial vacuum, so that most of the electrons can reach the electrode, without hitting too many air molecules.
The power of the ultraviolet source, typically a laser, depends on the current and the charging time of the conductors. For example, if a conductor has a capacitance of 100 pF, and if this conductor is to be positively charged to 100 V in one ms, then the corresponding current will be:I=Q V/t=100×10−10/0.001=10−5 A 
The necessary power of the laser must be:P=I×Eph/Efficiency=10−5×4.8/0.05≈mW assuming that 5% of the photons emitted effectively produce the photoelectric effect.
Now if the interconnection of substrates is to be checked, such as semi-conductor chips or printed circuits, all the conductors must be checked for any full or partial cut between two pads that are supposed to house components, with regard to the theoretical model, and each conductor must be checked to make sure that there is no dead or partial short circuit with one or several other conductors.
In previous practices (U.S. Pat. No. 4,967,152, for example), cuts are detected according to the following principle:
A fine metal grid is arranged vertically to the substrate to be tested. It is taken to a positive potential in relation to the substrate to be checked, for example in the order of 100 V.
The ultraviolet source passing almost transparently through the metal grid, is directed onto the end of a conductor and leads to the release of electrons from the corresponding conductor. These electrons are then captured by the grid, that attracts them. A current is then collected. This operation is continued until the current collected reaches a determined threshold, which means that the conductor is then electrically charged to the value of approximately 100 V. The charging time depends, among others, on the conductor's capacitance, and on the power of the laser.
A conductor previously charged in this way does not lead to the release of other electrons in the same conditions of excitation, and regardless of the place at which its excitation is done. On the other hand, if it is cut somewhere, and it the same excitation is done on the other side of the cut, then the part that has not previously been charged will emit of electrons in the form of a capacitance charge.
Then, the method is repeated to test the continuity of the other conductors.
However, in the partial vacuum used, the conductors do not discharge. Thus, at the end of the test described above, the substrate will be completely charged. Insofar as the laser source is not affected by the charge of the neighbouring conductors, that does not unduly disturb the photoelectric effect, but the electrons ejected have a tendency to go to join neighbouring conductors previously charged to 100 V, and no longer solely to the grid, which significantly alters the current collected by the grid, and distorts the conclusions drawn from the value of the currents measured, which can lead to an incorrect diagnostic.
One possible alternative is to gradually increase the voltage of the collection grid throughout the test, but, nonetheless, a loss of electrons is observed, the latter being collected by neighbouring conductors of potential (the last ones tested). Furthermore, as the number of conductors may be in the order of several thousands, the idea of increasing the potential by progressive jumps along the conductors is soon limited.
Moreover, the measurement of the resistance, an important criterion when validating the test principle of a technology, is not really possible here: even weak traces of conductors defectively linking the two portions of a single conductor will have a time constant RC that is sufficiently short not to be detected, while the corresponding resistance (high in this case) of the link should be analyzed like a real defect.
Short-circuits are detected in previous practices by first charging a given conductor, and by looking for other conductors having a similar potential on the substrate, highlighting a physical connection with the reference conductor, and thus the existence of a short circuit. Here, once again, there is the problem of conductors discharging.
The use of the photoelectric effect to measure short circuits and cuts on substrates having a large amount of conductors is therefore a problem.
In more recent previous practices, a single source of light is typically used to carry out circuit tests, which considerably reduces productivity and therefore increases the cost of them.
Contactless test systems have recently been produced. However, these systems cannot measure low interconnection resistances of certain substrates, such as printed circuits that are part of the new generations of boxes for semi-conductor chips, while the increasingly higher frequencies of this type of application require interconnection resistances lower than a threshold (typically 10 ohms or less). These resistances cannot be measured by the contactless test technologies of previous practices (electron beam test as described in the document U.S. Pat. No. 4,573,008 by using an electric conduction path, by creating a plasma or by using the simple photoelectric effect).
In fact, none of the technologies described above enables a current to be circulated from one end to the other of a conductor, without mechanically accessing (by touching it) one and/or the other of its ends, and none of them enables the resistance of these conductors to be measured accurately.